Over the last 100 years, petroleum fuels have dominated due to their high energy content, relative ease of storage, and their abundance. The age of abundant oil is ending, however, and while the new finds seem to be fewer and farther between, the demand for petroleum fuels has only increased. The coming scarcity is obvious to many, and while some debate how much oil still exists, nearly everyone agrees that we will reach a point when petroleum will no longer be a viable source of fuel. While there is agreement on the eventual end of petroleum fuels, there has not been any real consensus on what will be the new power source of all transportation. Many potential alternatives have been proposed, from corn-derived ethanol to hydrogen, but there are prohibitive problems with nearly all of these methods. One interesting alternative that has not received much attention, is the use of ammonia as a liquid fuel.
Ammonia, which has the chemical composition of NH3, can be combusted with oxygen to produce nitrogen gas and water. The reaction is exothermic, and while ammonia’s energy content is about half that of gasoline, the fact that its combustion produces only nitrogen gas and water makes it’s use as a fuel increasingly attractive as CO2 regulation becomes more likely.
I attended a symposium on offshore wind in Houston recently, put on by the Ocean Energy Institute, and one of the speakers spoke of the potential to use ammonia as a transportation fuel, and ammonia production as a means of energy storage. His proposal centers on using off-peak power from massive offshore wind turbines to run water desalinization plants, and then strip the hydrogen from the purified water to combine with nitrogen to create ammonia. The draw of this proposal is that it will make use of the typically unwanted off-peak power to produce pure water and ammonia, both of which are already in high demand today.
Is this too good to be true? Well, no, not really, but there are several hurdles that must be overcome before we are driving around in ammonia powered vehicles. First, while ammonia production is already over 150 million tones per year, most of that is produced through either natural gas, and it is mostly used for fertilizers. Since current ammonia production utilizes fossil fuels, using what we have now as a fuel would not really be any ‘greener’, so the first step is developing commercial-scale ammonia plants that require only electricity, water, and N2 (from air) as inputs. By using only these three inputs, all of which can be theoretically produced by renewable power, then the cycle becomes carbon-free. I believe this is not nearly as simple as it sounds, but I do think it is well within our capabilities as a society. While the production of ammonia would have to be scaled up by about a factor of 10, the up-side is that we already know and understand how to store and transport ammonia safely and economically. Ammonia is a gas at room temperature, but it does compress into a liquid at around 125psia, which makes it much more user-friendly than hydrogen gas. Ammonia can also be combusted in traditional internal combustion engines with only minor modifications required, which is a huge benefit considering there are over 250 million cars in the US alone.
Overall, I believe that without breakthroughs in electrical storage technologies, liquid fuels will remain the best choice for powering transportation vehicles. The portability, power density, ease of refueling, and ease of storage is what makes gasoline and diesel and other liquid fuels so popular, and why finding a sustainable fuel replacement is so vital. Is ammonia the fuel of the future? It is very possible, but only time will tell.